While RoonServer does not run on iOS, there’s nothing that really is stopping it from doing so. It’s just work to port over. The problem is that it would be a pretty foul experience.
The good news is that the latest iPad pros are comparable to core i7 processors from just a few generations ago. They should be able to be a capable server and even do demanding DSP without a hiccup. Even the latest iPhone is more powerful than most Roon Cores out there.
That said, iOS stinks at background processing, so you’d have to live in the “Roon Server” app. Additionally, your iPad would have to run at all times. I believe that includes having the screen on. If iOS decided to kill off Roon Server because you turned off the screen or switched to apps, you’d have to go through an entire boot up of the Roon server. That takes time because of all the in memory indexes.
There’s another problem that the primary networking of the iPad is via Wi-Fi. If your music is sitting on a NAS, that means putting quite a burden on your Wi-Fi network. Same goes for multi-zone or high-res streams. Yes, you could use a lightning to USB to ethernet adapter, but now you are starting to bolt down that iPad. You might as well get something far cheaper with more storage.
The most interesting part of this, is how quickly our perception of the tablet or phone being a weak device is becoming obsolete.